Regulation of calcium concentrations in synaptosomes: alpha 2-adrenoceptors reduce free Ca2+ by closure of N-type Ca2+ channels.

The relationship between intrasynaptosomal total (CaT) and free ([Ca2+]i) calcium and 45Ca accumulation was studied under physiological and K(+)-depolarised conditions in rat cortical synaptosomes. Under physiological conditions, CaT (10.7 mM) was approximately 10,000 times higher than [Ca2+]i (118 nM), showing that there is a large reservoir of sequestered calcium in synaptosomes. 45Ca accumulation was rapid (initial rate, 3.4 nmol/mg protein/min), substantial (7 nmol/mg protein in 2 min), and depolarisation dependent, and reached equilibrium after 5 min. At equilibrium, only 10% of CaT was freely exchangeable. This pool was much larger than the free Ca2+ pool. CaT, [Ca2+]i, and 45Ca accumulations were directly related to the Ca2+ concentration in the buffer, suggesting that [Ca2+]i is not highly conserved but is maintained by simple equilibria between the various pools. Clonidine reduced 45Ca accumulation in a time- and dose-dependent manner. Maximum inhibition (40% at 100 microM) occurred at 2 min and the IC50 was 80 nM. The reduction caused by clonidine (1 microM) reached equilibrium after 5 min, but this equilibrium value was lower than in controls, suggesting that clonidine changes the exchangeable Ca2+ pool size. The effects of clonidine (1 microM) on [Ca2+]i (26% reduction) and on 45Ca accumulation (24% reduction) were most apparent under physiological conditions. However, while it was not dependent on depolarisation, it did not occur in physiological buffer containing low K+ concentration (0.1-1 mM). The inhibitory effect of clonidine on 45Ca accumulation is receptor mediated as it was antagonised by idazoxan (1 microM).(ABSTRACT TRUNCATED AT 250 WORDS)